1. Field of the Invention
The present invention relates to a charge member, a charge apparatus that comprises the charge member, a process cartridge that comprises at least an image support member and a charge member, and to an image forming apparatus of a copier, printer or facsimile apparatus, and the like having the aforementioned charge member, charge apparatus, or process cartridge; and more particularly, relates to evaluation and judgment for determining the optimum gap between the image support member and the charge roller in order to conduct efficient charging.
2. Description of the Related Art
In image forming apparatuses that use an electronic photographic process, for example, after a visible image is formed by conducting the processes of charging, exposing and developing in relation to a photosensitive member, which is the image support member, the image is formed by using a transfer process to transfer the visible image on the photosensitive member to a transfer medium, and by using a fixing process to fix the image that was transferred onto the transfer medium.
In the past, scorotron charging devices were used in the aforementioned charging process to charge the photosensitive member, which is the member to be charged, but recently charge rollers have come to be used for the charge member in order to reduce the generation of harmful gases such as ozone and nitrous oxides (NOx) because of environmental concerns, and to allow the making of a more compact apparatus. In a charging mechanism using a charge roller for charging, discharge does not occur if the gap between the photosensitive member and the charge roller is too narrow, and the space on the Paschen side becomes 8 μm. However, the charge roller and the photosensitive member actually have a capacitance component, and therefore, discharge begins at 20 μm or more, and with a gap of 20 μm or more, the density of the discharge becomes smaller the wider the gap.
In the past, uniformity of resistance has been desired because if there were partial irregularities in the resistance of the charge roller, the charge would concentrate on the part with the lowest resistance value, and excessively large current would flow locally to generate charge irregularities. Moreover, with regard to the surface unevenness of the charge roller, in order to make it easier to concentrate discharge at the convex part it was desirable to have little surface roughness. In so-called contact charging that uses contact between the photosensitive member and the charge roller, discharge occurs when the gap of the region coming off the outer side a little from the nip becomes 20 μm or more. In order to allow the photosensitive member to charge up to a specific electric potential when charging by applying DC voltage to the charge roller, the only correct discharge opportunity for discharging to the photosensitive member from various points on the charge roller is the one instant when passing through the gap width along the Paschen side, and therefore, if the surface of the charge roller is uneven, irregularities of charge potential corresponding to that unevenness will be generated. Thus, a smooth charge roller that maintains a constant relative position between the charge roller and the photosensitive member has been sought.
Meanwhile, if AC voltage is superimposed on the DC voltage on the charge roller when charging, negative and positive discharging is repeated corresponding to the frequency, and because the charge potential is balanced with the value of the DC voltage applied, it is not always necessary for the charge roller surface to be smooth. For example, in Japanese Patent Application Laid-open No. 2000-75701, in an image forming apparatus that charges by having the charge roller and the photosensitive member make contact, studies were conducted on the unevenness of the photosensitive member and the charge roller, and on the positive addition of unevenness on the surface of the charge roller in order to form micro-spaces in the nip and generate discharge.
However, if the unevenness added to the surface of the charge roller is large, there is the possibility of damaging the photosensitive member because of contact between the photosensitive member and the charge roller, and therefore it is better to use a short-lived photosensitive member, but considering the durability of the photosensitive member, it is still preferable that the charge roller have a smooth surface.
Moreover, when forming an image by having the charge roller and the photosensitive member make contact, unless the toner remaining after transfer can be completely cleaned off, the remaining toner is caught between the charge roller and the photosensitive member, and causes irregularities of image concentration to occur by adhering to the charge roller and producing fluctuations in resistance, which brings about fluctuations of the charge potential of the photosensitive member. For that reason, as disclosed in Japanese Patent Application Laid-open No. 2004-264792, Japanese Patent Application Laid-open No. 2002-108059, and Japanese Patent Application Laid-open No. 2005-4000, so-called non-contact charging was proposed, in which the photosensitive member is charged by providing a gap between the photosensitive member and charge roller.
As previously described, because the density produced when discharging varies depending on the size of the gap, in contrast to contact charging, the gap between the photosensitive member and the charge roller must be accurately controlled in non-contact charging. For this reason, in the past a smooth shape was the ideal for the surface of the charge roller in non-contact charging. In this charging process, the photosensitive member is charged to the target voltage by simultaneously superimposing AC voltage when applying DC voltage to the charge roller. When AC voltage has been superimposed on the DC voltage on the roller, positive and negative discharge is repeated between the charge roller and the photosensitive member corresponding to the frequency, and the charge potential of the photosensitive member is equalized with the value of the DC voltage. The charging parameters at this time include the voltage and frequency of the alternating current applied, resistance irregularities of the charge roller, resistance irregularities of the photosensitive member, the gap between the photosensitive member and the charge roller, and gap variations. If there are gap variations, the discharge density varies depending on the gap, causing charge irregularities.
The dimensional precision of the photosensitive member and the charge roller, the installation precision, and vibration (fluctuation) may be cited as causes that produce gap variations. Of these, the dimensional precision of the photosensitive member and the charge roller may be raised to a precision unhindered by gap variations by setting suitable manufacturing conditions. Specifically, by studying the immersion coating conditions, the coating irregularities of the photosensitive member can be kept to under a few microns. Moreover, by increasing the strength of the spring and the precision of the charge roller or photosensitive member support member, it is also possible to reduce variations of installation precision enough so that discharge density variations do not become a problem.
On the other hand, reducing the vibration (fluctuation) of the photosensitive member is extremely difficult. Specifically, in order to suppress the vibration (fluctuation) of the photosensitive member, because the tube of the photosensitive member is comprised of a metal cylinder, it is necessary to make the aluminum, and the like cylindrical tube thicker, to heighten to an extreme the precision of the cylindrical tube and flange, and to raise the assembly precision of the cylindrical tube and flange. However, because the tube constitutes an extremely high percentage of cost of the photosensitive member, a very thick tube cannot be used in the photosensitive member. Moreover, because generally the flange is plastic, the photosensitive member is metal, and the flange is pressure fit to the cylindrical tube, there are limits to the precision in installing the photosensitive member drum and the flange, and normally the circumferential fluctuation can only be kept to about an average of 10 μm. Specifically, fluctuation can be minimized by selecting only photosensitive members with small fluctuation and making the flange out of metal, but these methods greatly heighten the cost of the photosensitive member. Here, fluctuation occurs even in the charge roller, but because the charge roller comprises a metal cylindrical column with a small external diameter, charge roller fluctuation can be ignored in relation to the vibration of the photosensitive member.
When charging a photosensitive member that fluctuates, the larger the frequency of the AC voltage applied to the charge roller, the higher the discharge density and the possibility of making the charge potential of the photosensitive member uniform. Nonetheless, if the frequency is too great, the photosensitive member and charge roller deteriorate faster, and therefore it is desirable to set the frequency as low as possible.
As indicated above, in order to prevent the occurrence of charge irregularities as much as possible, it is important to raise the dimensional precision of the photosensitive member and the charge roller, to raise the installation precision, and to lower vibration (fluctuation), which are the causes that generate variations in the gap between the charge roller and the photosensitive member. However, suppressing the deterioration of the photosensitive member and the charge roller, and reducing charge irregularities at low cost posed big problems for the prior art because of heightened production costs for the photosensitive member.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Application Laid-open No. S52-36016, Japanese Patent Application Laid-open No. H09-311526, and Japanese Patent Application Laid-open No. 2004-038056.